f: slice optimization

Author: arik-so

lightning/src/ln/msgs.rs

@@ -2777,16 +2777,21 @@ impl Writeable for OutboundTrampolinePayload {
 				});
 			},
 			Self::LegacyBlindedPathEntry { amt_to_forward, outgoing_cltv_value, payment_paths, invoice_features } => {
-				let mut blinded_path_serialization: Vec<u8> = Vec::with_capacity(2048);
-				for current_payment_path in payment_paths {
-					current_payment_path.inner_blinded_path().write(&mut blinded_path_serialization)?;
-					current_payment_path.payinfo.write(&mut blinded_path_serialization)?;
-				}
+				let mut blinded_path_serialization = [0u8; 2048]; // Fixed-length buffer on the stack
+				let serialization_length = {
+					let mut blinded_path_slice = &mut blinded_path_serialization[..];
+					for current_payment_path in payment_paths {
+						current_payment_path.inner_blinded_path().write(&mut blinded_path_slice)?;
+						current_payment_path.payinfo.write(&mut blinded_path_slice)?;
+					}
+					2048 - blinded_path_slice.len()
+				};
+				let blinded_path_serialization = &blinded_path_serialization[..serialization_length];
 				_encode_varint_length_prefixed_tlv!(w, {
 					(2, HighZeroBytesDroppedBigSize(*amt_to_forward), required),
 					(4, HighZeroBytesDroppedBigSize(*outgoing_cltv_value), required),
 					(21, invoice_features.as_ref().map(|m| WithoutLength(m)), option),
-					(22, WithoutLength(&blinded_path_serialization), required)
+					(22, WithoutLength(blinded_path_serialization), required)
 				});
 			},
 		}